FIG. 6 is a view exemplifying a small motor with a gear unit which has been generally used conventionally, FIG. 6A is a general view for the small motor with a gear unit, showing the gear unit portion in a partial sectional view, and FIG. 6B is a view exemplifying the gear unit portion shown in a partial sectional view as seen from a direction different from FIG. 6A. Also, FIGS. 7A and 7B are views exemplifying a C-type snap ring as seen from directions different respectively, and FIG. 7C is an enlarged view showing an essential part of FIG. 6B. As shown in FIG. 6, a motor portion is installed to the gear unit portion through the use of a screw or the like. The gear unit portion has, within a gear box made of resin, a worm fixed near the tip portion of a motor shaft extended from the motor portion, a helical gear engaging with this worm, and the output shaft for taking out driving torque to the outside from this helical gear. Such a small motor with a gear unit can be used for driving, for example, an automotive power window.
The worm can be fixed by, for example, forming a knurled portion on the motor shaft, and providing the worm with a worm fitting hole for fitting in it to press the worm into the motor shaft. Since a helical gear for constituting a worm wheel is engaged with this worm, the driving torque outputted from the motor portion is transmitted from the motor shaft to the worm, is transmitted from the worm to the helical gear in the gear unit portion, and is taken out form the output shaft to the outside. Since in the output shaft and the helical gear, mutual movement in a direction of rotation is regulated via shock absorbing rubber, they are constructed so as to rotate, with the fixed shaft fixed to the gear box as a center, around it in a body.
Conventionally, the fixed shaft of the gear box has been constructed by a solid shaft obtained by cutting steel material or the like, and has been fixed in a cantilever state by pressing in to a gear box made of resin or insert molding or the like. At this time, in order that the fixed shaft is prevented from coming off from the gear box, the end portion of the fixed shaft which is fixed to the gear box by pressing-in or insert molding has been subjected to dislocation preventive work such as knurling work.
Also, in order to prevent a part such as the helical gear and the output shaft from coming off from the fixed shaft in a thrust direction, a groove has been cut around the fixed shaft in the vicinity of the tip side (the other side of the fixed side) of the fixed shaft, and in this groove, a C-type snap ring or the like has been fitted for fixing.
Since it has been obtained by cutting steel material or the like as described above, the conventional fixed shaft has had drawbacks that it is high in machining cost, heavy in weight because of a solid shaft, and the material cost is also expensive. In the vehicle motor, there is a request for weight reduction from fuel economy and environmental problems. Also, in order to prevent corrosion, surface treatment such as plating has been required after machining. Further, in order to prevent the helical gear, the output shaft and the like from coming off in the thrust direction, it is necessary to groove the fixed shaft and fix through the use of a C-type snap ring or the like, and therefore, the number of parts is increased, it takes time to assemble, and the cost is increased.
[Patent Literature 1] Japanese Patent Laid-Open No. 6-38449